Japanese Patent No 3901928 (referred to as '928 patent, hereafter) discloses a controller for an electric power steering device comprising controlling means for controlling driving a motor based on a motor current instruction value, said controller comprising target steering torque setting means for setting a target steering torque based on a steering torque and a vehicle speed; assist current calculating means for calculating an assist current instruction value based on a steering torque, the target steering torque and the motor current of the motor; target steering angle setting means for setting a target steering angle for returning the steering wheel back to a neutral position based on the steering angle and the vehicle speed; a target steering angular velocity setting means for setting a target steering angular velocity based on a deviation of the steering angle from the target steering angle and the vehicle speed; target converging current setting means for setting a target converging current based on a deviation of the steering angular velocity from the target steering angular velocity; and determining means for determining whether a user releases his/her hands from the steering wheel based on the steering torque, wherein the output of the target converging current of the target converging current setting means is made valid or restrained according to the determination result of the determining means, and the output is added to the assist current instruction value to generate the motor current instruction value.
According to the '928 patent, if the situation in which the user releases his/her hands from the steering wheel is detected, the target converging current set based on the deviation of the steering angle from the target steering angle for returning the steering wheel back to a neutral position is added to the assist current instruction value to generate the motor current instruction value. Thus, even under the situation in which the steering torque is not generated, a torque corresponding to the target converging current for returning the steering wheel back to a neutral position is additionally generated, which improves a capability of returning the steering wheel to the neutral position.
In order to reduce necessary power for maintaining the steering angle and improve stability in maintaining the steering angle, it is possible to mechanically apply a friction torque to a steering system. However, since an appropriate friction torque differs depending on a status of the vehicle such as a vehicle speed, a steering angle, etc., it is desirable to generate the friction torque not by means of a mechanical mechanism but by means of electric control.
With respect to this point, according to the '928 patent, the friction torque is generated using the target converging current by means of electric control, though such a situation is limited to a specific situation in which the user releases his/her hands from the steering wheel. However, since the friction torque is generated as a function of a steering velocity (steering angular velocity), it is not possible to reproduce static friction. Specifically, since the steering velocity is zero in a static state, it is not possible to generate the friction torque in such a static state. For this reason, according to the '928 patent, it is not possible to reduce the power required to maintain the steering angle and improve stability in maintaining the steering angle, and it is not possible to reduce steering pull in traveling straight forward, either.